Either pay a cloud storage provider about $4/month to hold it for you, or buy a couple external drives and verify your backups yourself about once a year then replace drives as they eventually fail.
I figure the break even point is about two and a half years. If you're committed to hoarding your content for longer than that, buy storage drives. If not, rent cloud storage. If your drives die in less than that you would have been better with cloud storage, but that isn't likely.
I don't have a NUC. I have the BeeLink Mini S12 I linked. It has room for the 2TB NVMe and the 4TB SSD I have in it. It doesn't have any space for any more internal drives. You could plug a USB enclosure into it but I wouldn't because I want to keep most of my storage attached to my desktop and not on my network so I don't intend of ever adding any more storage to it. If you want more network storage you should consider a physically larger machine.
It runs Alpine Linux and several Docker services. I use a samba network share for NAS features, but I also have Seafile running.
Markdown is plain text; it just adds some formatting syntax. It is on your spreadsheet on row 347. You can read about it here.
You can get HDDs at under $15/TB while SSDs can be over $50/TB. Option 1 is more than just overpaying; it is a waste of money. SSDs are great, but they are not for bulk data storage. With that said I do own a few 4TB SSDs and a few 2TB NVMes, but none are for bulk data storage. They mostly hold the OS and my installed games and the working set of files I'm actively using.
Option 2 is good, but can probably be improved upon.
Option 3 is what I do and recommend to you. I'll explain next.
Option 4 is the same as Option 2, but you pay more for the out-of-the-box convenience. It isn't for me, but plenty of people like Synology, so no judgement here if that is for you.
Your two real options represent a dilemma so many people encounter: Do I want an external enclosure, or do I want a NAS?
I use a 5 bay external enclosure plugged into my desktop PC. I use DrivePool to pool the 5 drives together and I use its duplication feature for redundancy. It has 5x12TB refurbished Seagate Exos in it. If I need more space I'll probably replace some 12TB drives with larger 18 or 20TB drives. I keep it powered off when not in use and I am the only user. I may never need more space than 5x20TB drives can offer and I think it is the best option for my use case. It sounds like your needs are similar to mine.
I'm also interested in network storage and server services, but I don't need all of my storage available on my network. I have a small mini pc with 6TB of storage on it and it is all I need for a NAS or home server. 6TB is plenty for temporary storage and streaming videos.
So, my external enclosure was $200 and the mini pc was $200 (but $337 for both today, Cyber Monday). You could build a NAS for $400 with the same N100 processor and a case with enough space for the drives. The difference between that and my setup is I can turn the external enclosure off while the mini continues to run 24/7 and it uses very little electricity. However, only a small subset of my data is available on my network. There are tradeoffs either way. I can wipe and reinstall Linux on my mini without impacting my bulk storage, for example. My mini is silent and small enough to take with me on a vacation to stream movies; it would be too big for that if it had a bunch of HDDs in it, however I can stream videos from it from anywhere in the world using Tailscale already, so no good reason to bring it anyways.
You'll have to decide which you'd prefer, but I'm happy with my external enclosure and mini combo. I doubt I'll ever build a tower desktop full of HDDs ever again.
Replace your low capacity drives with higher capacity drives.
Seafile has SeaDrive which mounts like an external drive and works just like other similar services like OneDrive.
A mirror is not a backup, like you've noticed. Use rsync for your backups if you don't care about versioning or snapshots. You can run it on a schedule to copy files to your backup, but not delete anything. Then you can manually run it with the delete option turned on when you're ready.
That and https://filebrowser.org is free.